Process of preparing gamma-terpinene diadducts

ABSTRACT

Diadducts of gamma-terpinene and Diels-Alder dienophiles are prepared in high yield by contacting dienophile and gamma-terpinene in a preferred molar ratio of at least about 2:1 at elevated temperatures. The resulting reaction product generally comprises at least about 80% diadducts and no more than about 20% monoadducts. Preferred dienophiles include maleic anhydride.

BACKGROUND OF THE INVENTION

This invention relates to the preparation of Diels-Alder dienophilediadducts of gamma-terpinene, and more specifically, to the high yieldpreparation of maleic anhydride diadducts of gamma-terpinene.

It is well known that Diels-Alder dienophile monoadducts of terpenes,which are here defined as molecules which result upon covalent joiningof one molecule of a Diels-Alder dienophile and one molecule of aterpene, can form upon contact of Diels-Alder dienophiles and terpenesat elevated temperatures. Minor amounts of diadducts, which are heredefined as molecules in which there is a covalent joining between twomolecules of a Diels-Alder dienophile and one molecule of terpene, canalso form upon contact of Diels-Alder dienophiles and terpenes atelevated temperatures. For example, U.S. Pat. No. 1,993,031, issued toPeterson, discloses that an amorphous resinous compound may be producedby reacting maleic anhydride, a typical Diels-Alder dienophile, with aterpene cut comprising terpenes having a non-conjugated system of doublebonds. Such terpenes are disclosed as being alpha-pinene, beta-pinene,terpinolene, and d-, 1- and d,l-limonene. All of these terpene isomersare readily available by the distillation of turpentine or orange oil,and it is now known in the art that their reaction with maleic anhydridegives a product in which the major component is monoadduct.Gamma-terpinene comprises a minimal portion of turpentine and is notcited in the bulk of this literature.

Despite a wealth of literature relating to the reactions betweenterpenes and maleic anhydride, there has long been a need for a processby which substantial yields of maleic anhydride diadducts of terpenescould be obtained. High yield methods for preparing such diadducts areespecially desirable, since the process of separating undesirablemonoadducts from diadducts is costly and time-consuming. In an attemptto provide such a process, Schluenz et al, in U.S. Pat. No. 4,107,420,proposed reacting a non-conjugated terpene and maleic anhydride,preferably in equimolar amounts, at temperatures between 140° C. and200° C. in the presence of 0.002 to 0.03% iodine, based on the weight ofthe terpene. Schluenz et al claimed that by using this process theycould prepare a mixture containing more than 15% diadducts; theirexamples show the preparation of reaction products containing up to 45%diadduct. Terpenes disclosed as being useful in the Schluenz et alprocess include limonene, terpinolene, terpineol, 1,8-cineole,1,4-cineole, and gamma-terpinene (1,4-para-menthadiene). However,gamma-terpinene is not actually utilized in any of the providedexamples. Nor do they claim their process produces a specific diadduct,for example, the diadduct of gamma-terpinene. Rather their processproduces a complex mixture of isomeric diadducts.

Although Schluenz et al disclose and claim the use of iodine in theirprocess, Example 4 in their patent illustrates the process run in theabsence of iodine using terpinolene as the terpene. The resultingreaction product comprises about 45% of diadduct, a yield essentiallythe same as that obtained in the presence of iodine. However, theproduct obtained in the absence of iodine is dark yellow in color ratherthan pale yellow. The Schluenz process, then, apparently improves thecolor of the product but not the yield of diadduct.

U.S. Pat. No. 4,670,504 to Cardenas discloses a tackifier compositionformed from a resin which is preferably a polymerized terpene thatsubsequent to formation is reacted with an acid or anhydride such asmaleic anhydride. It is disclosed that gamma-terpinene is one of manypossible terpenes which can be used to prepare the polyterpene. Theanhydride is said to become chemically incorporated with the polyterpenein a manner which does not appear to be fully understood. The additionof maleic anhydride to a pre-existing terpene polymer is to bedistinguished from the present invention in which maleic anhydride isadded to terpene monomer.

Diels-Alder dienophile diadducts of gamma-terpinene are expected to findextensive commerical use. For example, maleic anhydride diadducts willlikely be employed in alkyd formulations and as curing agents forepoxies and the like. Adduction between maleimides or bismaleimides andgamma-terpinene may lead to novel polyimides and/or their precursors.The product polyimides are expected to be useful composite componentsfor high temperature structural applications. So called "mixed"Diels-Alder dienophile diadducts of gamma-terpinene, in which onemolecule of gamma-terpinene is covalently joined with one mole each oftwo different Diels-Alder dienophiles, such as maleic anhydride andacrylic acid, are expected to be useful as well.

Therefore, it is an object of this invention to provide diadducts ofgamma-terpinene and maleic anhydride, maleimides, and acrylates,respectively.

It is another object of this invention to provide "mixed" diadducts ofgamma-terpinene.

It is a further object of this invention to provide diadducts ofgamma-terpinene and various Diels-Alder dienophiles.

It is still another object of this invention to provide diadducts ofgamma-terpinene and various Diels-Alder dienophiles without appreciableformation of monoadducts.

It is yet another object of this invention to provide diadducts ofgamma-terpinene and various Diels-Alder dienophiles in a moreeconomically desirable manner than previously known methods.

SUMMARY OF THE INVENTION

It has now been found that gamma-terpinene cleanly and readily reactswith Diels-Alder dienophiles to give a diadduct in high yield, i.e., inmajor proportion and commonly 80% or more. By contacting dienophiles andqamma-terpinene, preferably in molar ratios of about 2:1 , attemperatures in the range of about 140° to 200° C., a reaction productis obtained which can comprise at least about 80% diadduct and generallyno more than about 20% monoadduct. Gamma-terpinene presently appears tobe unique among terpene molecules in that no other material has beenfound to yield such high proportions of diadduct. Unlike the prior artprocess disclosed in Schluenz et al, the process of this invention isrun in the absence of iodine. The resulting crude reaction product is apale yellow solid and does not suffer from high coloration.

The gamma-terpinene/maleic anhydride diadducts of this invention areuseful, for example, as curing agents for epoxies and the like. Priorart reaction products containing predominately monoadduct are inferiorfor use as alkyd components and curing agents since the monoadducts actas reaction chain-stoppers rather than as cross-linking agents and thus,for example, lead to cured epoxy materials with relatively low heatdistortion temperatures. In order for such high monoadduct-containingproducts to provide optimum properties, they must first be subjected tocostly and time-consuming purification steps to separate out the desireddiadducts. By contrast, the reaction products of this invention, byvirtue of their high diadduct content, may be more easily purified ormay even be used directly as cross-linking or curing agents for alkyd orepoxy resin systems.

The term "terpene" covers a wide variety of compounds containing tencarbon atoms, some of which are cyclic and unsaturated. Among the latterare a group called menthadienes, which are monocyclic C₁₀ H₁₆hydrocarbons having their carbon atoms arranged as in p-isopropyltoluene and limonene. The terpene compound from which the diadducts ofthis invention are prepared is gamma-terpinene, also referred to as1,4-paramenthadiene, a non-conjugated diene having formula 1.Gamma-terpinene is not found in appreciable amounts in turpentine but isinstead a product of the acid catalyzed isomerization of turpentine. Itis obtained after such an isomerization process by careful fractionaldistillation of the isomerizate.

In preferred embodiments of this invention, gamma-terpinene is contactedwith a Diels-Alder dienophile at a temperature within the range of about50° to 200° C., preferably about 155° to 170° C. RepresentativeDiels-Alder dienophiles include maleic anhydride, maleimides,acrylonitrile, acrylic acid, methyl acrylate, and various chemicalmoieties, including acrylic acid derivatives. Maleic anhydride isparticularly preferred. As will be appreciated by one of skill in theart, a wide variety of Diels-Alder dienophiles (which are known per se)may advantageously be employed. The order of addition of the dienophileand gamma-terpinene is not believed to be critical to the practice ofthis invention. For best results, it is desired that the reactantscomprise at least about two moles of dienophile for each mole ofgamma-terpinene.

The lowest temperature at which the reaction will spontaneously occur isabout 155° C. with the preferred temperature range beginning at about160° C. The upper limit of the temperature range is generally determinedby the boiling points of the reactants, generally about 170° at 1atmosphere of pressure. At temperatures above about 165° C., thediadduction between gamma-terpinene and, for example, maleic anhydrideoccurs immediately upon contacting the reactants. It is best not topermit the temperature to fall substantially below about 155° C.because, in the case where terpene is being added to maleic anhydride,unreacted terpene tends to accumulate, allowing for a very exothermicreaction once sufficient heat is provided to begin rapid reaction.

The reaction is preferably run without solvent, but solvents such asaliphatic or aromatic hydrocarbons, esters or ethers can be used, solong as it is both inert with respect to the adduction reaction and hasa boiling point greater than about the reaction temperature.Representative inert organic solvents include p-cymene (a common terpeneimpurity of gamma-terpinene), 2-methoxyethyl ether, and otherpolyethers. It is preferred that the gamma-terpinene feedstocks be ofgood purity, at least about 75%, as any olefinic terpene impuritiesreact to give mainly undesirable monoadducts.

It is believed that when gamma-terpinene and, for example, maleicanhydride are combined, an initial "Ene" reaction occurs resulting inthe covalent joining of one terpene and one maleic anhydride unit and ashift of one double bond so that a conjugated system of double bondsresults. These conjugated double bonds are then believed to immediatelyundergo a Diels-Alder reaction with additional maleic anhydride at thetemperature required for the Ene reaction. The product is found to be aterpene twice adducted with maleic anhydride. This Ene/Diels-Alderaddition is presently believed to be unique to gamma-terpinene amongpara-menthadienes. This reaction sequence is shown in the schematic,wherein the addition of the first mole of maleic anhydride is shown inbrackets and then the products obtained upon the addition of the secondmole of maleic anhydride are set forth as formulas 2, 3, 4, and 5.##STR1##

Unlike the prior art Schluenz et al process described in U.S. Pat. No.4,107,420, the process of this invention is carried out without theaddition of iodine. Contrary to what would have been expected in view ofthe disclosure of the Schluenz et al patent, distinctly differentproducts are obtained in the absence of iodine and in the presence ofiodine. The diadduct yield of the process of this invention is almosttwice as much as the yield of any of the Schluenz et al diadducts. Thereaction product of the process of this invention consists largely ofonly about 4 isomers (represented by formulas 2-5) and also generallyhas light color; measured on the Gardner scale, the color of thisreaction product is typically about 1.

The invention will now be described, and further compared to theSchluenz et al process, in the following examples wherein parts andpercents are by weight and temperatures are in degrees Celsius unlessotherwise indicated.

EXAMPLE 1

A reaction flask was set up with necks fitted with a mechanical stirrer,an addition funnel, temperature probe, and a reflux condenser toppedwith a nitrogen inlet which maintained in inert atmosphere in thereaction flask. To the reaction flask containing 104g of maleicanhydride at 155° was added 75g of gamma-terpinene having 98% purity.The addition took 25 minutes and the temperature fluctuated between 153°and 165°. Upon cooling, the product mixture was Kugelrohr distilled toremove unreacted terpene, maleic anhydride and monoadducts. The purifiedproduct was obtained in 87% yield based on total charge of reactablemonomers. The product contains about 95% diadduct and 5% monoadduct asjudged by gel permeation chromatography using a refractive indexdetector. The product has a neat Gardner color of 2, and a 50% by weightsolution in tetrahydrofuran has a Gardner color of 1. The product willoccasionally have a yellow or red tint. The ring-and-ball softeningpoint of the product was 97° C.

COMPARATIVE EXAMPLE 1

The same general process taught by Schluenz et al in U.S. Pat. No.4,107,420 for preparing terpene/maleic anhydride diadduct was repeatedusing 98% pure gamma-terpinene. Using the same apparatus as described inExample 1, the flask was charged with 98.2g maleic anhydride (1 mole)and heated to a temperature of 175° C. A crystal of iodine weighing0.03g was added, and then 136g (1 mole) of gamma-terpinene was drippedin slowly. After addition was complete, a sample analyzed by gelpermeation chromatography indicated that a diadduct was present only in9% yield, monoadduct was present in 87% yield. Unreacted terpenesconstituted 4% of the product mixture.

The procedure was repeated using maleic anhydride which was dried byazeotropic distillation from toluene/maleic anhydride; a similar productdistribution was obtained.

COMPARATIVE EXAMPLE 2

The procedure of Schluenz et al was repeated except no iodine was used.A product mixture consisting of 60% diadduct, 23% monoadduct and 18%unreaCted terpenes was obtained. This represents a molar yield ofdiadduct of over 84% based on available maleic anhydride.

What is claimed is:
 1. A method for preparing diadducts of a Diels-Alderdienophile and gamma-terpinene which comprises heating in the absence ofiodine a mixture of gamma-terpinene and about two molar equivalents ofthe Diels-Alder dienophile based upon the moles of gamma-terpinene at atemperature between about 155° C. and the boiling point of the mixturefor a time sufficient to react substantially all of the gamma-terpinene.2. A method for preparing diadducts of a Diels-Alder dienophile andgamma-terpinene which comprises heating one molar equivalent ofgamma-terpinene in the absence of iodine to a temperature at whichgamma-terpinene will undergo an "Ene" reaction with the Diels-Alderdienophile, adding about two molar equivalents of the Diels-Alderdienophile, and maintaining the temperature until substantially all ofthe gamma-terpinene has reacted.
 3. A method for preparing diadducts ofa Diels-Alder dienophile and gamma-terpinene which comprises heating inthe absence of iodine two molar equivalents of the Diels-alderdienophile to a temperature sufficient for the Diels-Alder dienophile toundergo an "Ene" reaction with gamma-terpinene, adding one equivalent ofgamma-terpinene and maintaining the temperature until substantially allof the gamma-terpinene has reacted.
 4. The method of claim 1 wherein theDiels-Alder dienophile is maleic anhydride and the temperature isbetween about 155° C. and about 170° C.
 5. The method according to claim1 wherein the Diels-Alder dienophile is maleic anhydride.
 6. The methodaccording to claim 3 wherein the Diels-Alder dienophile is maleicanhydride.
 7. The method of claim 1 wherein said mixture furthercomprises an inert organic solvent having a boiling point greater thanabout 160° C. at 1 atmosphere pressure.
 8. The method of claim 1 whereinto said gamma-terpinene is added a part of a blend of terpenes andcomprises at least about 75 mole percent of the total amount of terpenespresent in said blend.
 9. The method of claim 4 further comprisingrecovering, in admixture, compounds of the formulas ##STR2## from thereaction mixture.
 10. The method of claim 1 wherein said diadducts areformed in major proportion from the reaction.
 11. The method of claim 1wherein said diadducts comprise at least about 80% by weight of thereacted mixture.